Electrical device comprising metal oxide-containing solid electrolyte and electrode



Nov. 12, 1968 l.. FULLMAN ET AL 3,410,728 ELECTRICAL DEVICE COMPRISINGMETAL OXIDE-CONTAINING SOLID ELECTROLYTE AND ELECTRODE Filed June 10.1965 /n Venter-.s Rober-- L. Ful/man; Sthdn 7? M/tof by His r--rv/zf.`

United States Patent O ELECTRICAL DEVICE COMPRISING lVlElFAliJOXIDE-CUNTAINING SOLID ELECTROLYTE AND ELECTRODE Robert L. Fullman,Schenectady,

Elnora, N.Y., assignors to a corporation of New York Filed June 1t),1965, Ser. No. 462,852

4 Claims. (Cl. 136--86) and Stephan l. Mitolf, General Electric Company,

ABSTRACT F THE DISCLOSURE This invention relates to high temperaturefuel cells, and more particularly to composite articles providingelectrode-electrolyte-electrode structures or electrolyteelectrodestructures for such high temperature fuel cells.

Fuel cells, operable at high temperatures in the range of 1000" C. to1200 C. are shown in US. Letters Patent 3,138,487 and 3,138,490 whichare assigned to the same assignee as the present application. Each ofthese fuel cells employs a solid oxygen-ion conducting electrolyte,solid electrodes, fuel and oxidant supplies for the respectiveelectrodes, and electrical leads connected to the respective electrodes.Such fuel cells provide a low voltage direct current power source on acontinuous basis. Such cells have application in various chemicalprocess industries, such as the manufacture of aluminum and theelectro-refining of copper. Furthermore, these cells can be employed tooperate direct current motors.

In a fuel cell of the above type, it would be desirable to minimize theamount of silver employed as the cathode; to minimize the exposedsurface area of the silver, and to provide an electrode which functionsas either a cathode or anode. The present invention is directed to animproved composite article providing an electrode-electrolyte-electrodestructure or an electrolyte-electrode structure for a high temperaturefuel cell.

It is an object of our invention to provide an improved compositearticle forming an electrode-electro]yte-electrode structure for a hightemperature fuel cell.

It is another object of our invention to provide an improvedcompositearticle forming an electrolyte-electrode structure for a hightemperature fuel cell.

It is a further object of our invention to provide an improved hightemperature fuel cell which employs an improved composite article.

In carrying out our invention in one form, a composite article comprisesa solid oxygen-ion conducting member, and a non-porous adherentelectrode on one surface of the member, the electrode consisting of anoxygen-ion conducting metal oxide, and at least partially dissolvedtherein, a metal oxide selected from the group consisting of iron oxide,manganese oxide, cobalt oxide, vanadium oxide, titanium oxide, chromiumoxide, zinc oxide, titanium oxide-iron oxide, `and zinc oxide-ironoxide.

These and various other objects, features and advantages of theinvention will be better understood from the following descripton takenin connection with the` accompanying drawing in which:

FIGURE 1 is a sectional view of a composite article embodying ourinvention;

ICC

FIGURE 2 is a sectional view of a modified composite article;

FIGURE 3 is a sectional view of another modified composite article; and

FIGURE 4 is a sectional view of a high temperature fuel cell whichemploys a pair of solid electrodes embodying our invention.

In FIGURE 1, a composite article or body is shown generally Vat 10 whichcomprises a solid oxygen-ion conducting electrolyte member 11 in theform of a hollow tubular member of stabilized zirconia, and a pair ofsolid electrodes 12 -adhering tightly on opposite surfaces ofelectrolyte 11. Each electrode, which is preferably nonporous, consistsof an oxygen-ion conducting metal oxide and, at least partiallydissolved therein, `a metal oxide selected from the group consisting ofiron oxide, manganese oxide, cobalt oxide, vanadium oxide, titaniumoxide, chromium oxide, zinc oxide, titanium oxide-iron oxide, and zincoxide-iron oxide.

While both of the above electrodes 12 are described above as beingidentical, one of these electrodes is employable 'as the anode and adifferent cathode is provided as a tightly adherent layer on theopposite surface of electrolyte 11. For example, the cathode consists oflithiated nickel oxide, doped tantalum pentoxide, or a solid, porousoxygen-ion conducting metal oxide matrix with silver impregnated in andfilling the pores thereof. If an electrode 12 is used as the cathode,another anode material is employable therewith. For example, the anodeconsists of at least 50 volume percent of nickel and the balance beingan intimate dispersion of a compatible solid oxygenion conductingmaterial; or a solid oxygen-ion conducting metal oxide matrix, andsilver impregnated in and filling the pores thereof. The cathode oranode is positioned on either the inner or outer surface of electrolyte11.

In FIGURE 2 of the drawing, there is shown a modified composite articleor body 13 in the form of a container which comprises a solid oxygen-ionconducting electrolyte member 14, and a pair of solid electrodes 15adhering tightly on opposite surfaces of electrolyte 14. Each electrode15 has the same composition as electrodes 12 in FIGURE 1 of the drawing.

In FIGURE 3 of the drawing, there is shown another modified compositearticle 16 in the form of a plate. Article 16 comprises a solidoxygen-ion conducting electrolyte member 17, and a pair of solidelectrodes 18 adhering tightly on opposite surfaces of electrolyte 17.The composition of each electrode 18 is identical with the compositionof electrodes 15 in FIGURE 2 and electrodes 12 in FIGURE 1 of thedrawing.

In FIGURE 4 of the drawing, there is shown a high temperature fuel cell19 which includes composite article 10 of FIGURE 1 of the drawing.Composite article 10 comprises a solid oxygen-ion conductingelectrolyteI 11 in the form of a hollow tubular member of stabilizedzirconia, and a pair of solid electrodes 12 adhering tightly on oppositesurfaces of electrolyte 11. Electrodes 12 shown in FIGURE 4 areidentical in composition with electrodes 12 as shown in FIGURE 1 of thedrawing. Electrode 12 on the exterior surface of electrolyte 11functions as a cathodewhile electrode 12 on the interior surface ofelectrolyte 11 functions as the anode. An outer, hollow member 20 suchas a tube of alumina surrounds and is spaced from the exterior surfaceof cathode 12 to provide an air passage between cathode 12 and the innersurface of tube 20. A cover 21, for example, the same materials as tube20, is provided at the inlet end of tube 20.

An inlet tube 22 extends into the air passage between cathode 12 andtube 20 to introduce a gaseous oxidant Containing molecular oxygen froma source (not shown) into this passage. A second tube 23 is providedthrough cover 21 and communicates with the space defined by the interiorwall of anode 12 within electrolyte 11. Tube 23 introduces a fuel, suchas hydrogen, from a source (not shown) into this space. A conductingmetallic lead 24, for example, of nickel, extends through cover 21 andis in contact with anode 12. A conducting metallic lead 25, for example,of platinum of palladium, extends through cover 21 and is in contactwith cathode 12 of the cell. The free ends of leads 24 and 25 areconnected to apparatus, such as an electric motor (not shown), beingoperated by the cell. While both electrodes of this cell are shown anddescribed as having the above identical electrodes to provide a suitablehigh temperature fuel cell, a different cathode or anode is suitable forthis cell as was described previously.

A very satisfactory composite article for a high temperature fuel celloperable above 600 C. is provided by a solid oxygen-ion conductingmember with one or both electrodes consisting of an oxygen-ionconducting metal oxide and, at least partially dissolved therein, ametal oxide selected from the group consisting of iron oxide, manganeseoxide, cobalt oxide, vanadium oxide, titanium oxide, chromium oxide,zinc oxide, titanium oxide-iron oxide, and zinc oxide-iron oxide.

We found that such an electrode is a mixed conducting oxide electrodewhich provides both ionic and electronic conductivity. The oxygen-ionconducting metal oxide provides the ionic conductivity while solution init of a metal oxide lfrom the above group provides the electronicconductivity. The preferred electrode structure is non-porous. However,the porosity of the electrode structure is not critical to itsoperation. The improved composite article of our invention is employablein the form of a hollow tubular member, a flat plate or a container.

From the above metal oxide group, we prefer to employ iron oxide forelectronic conductivity in our electrode. We found 'further that thepreferred range for iron oxide, which includes Fe304, FeO and Fe2O3, insuch an electrode is from 2 weight percent to 20 weight percent Fe3O4,or an equivalent amount of iron introduced as Fe203, or FeO in theelectrode. The preferred oxygen-ion conducting metal oxide in ourelectrode structure and for our electrolyte member is solid stabilizedzirconia. However, other solid oxygen-ion conducting metal oxides suchas doped thoria are satisfactory for incorporating the electronicallyconductive metal oxide therewith.

Solid stabilized zirconia, which is a solid oxygen-ion conductingmaterial or oxygen-ion conducting metal oxide, is a compound with acubic crystal structure consisting of zirconia to which is added atleast one or a combination of several specific oxides such as calciumoxide, yttrium oxide, or a mixture of rare earth oxides. For example, asuitable solid zirconia material is stabilized with 14 molecular percentcalcium oxide. Other compositions of stabilized zirconia, which areemployable for the oxygen-ion member and as the oxygen-ion conductingmetal oxide in the electrode, are discussed in Oxide Ceramics byRyshkewitch, Academic Press, 1960, particularly on pp. 354, 364 and 376thereof.

Solid doped thoria is also a solid oxygen-ion conducting metal oxidewhich consists of thoria to which is added at least one or a combinationof several specific oxides such as calcium oxide, yttrium oxide, or amixture of rare earth oxides. For example, a solid doped thoria consistsof thoria which is doped with the addition of 4 molecular percentcalcium oxide to increase its oxygen-ion conductivity.

An eicient stable fuel cell is constructed which comprises a solidoxygen-ion conducting material as the electrolyte, an electrode incontact with one surface of the electrolyte, means for supplying agaseous oxidant containing molecular oxygen to the ele-ctrode, a secondelectrode in contact with the opposite surface of the electrolyte, meansfor supplying a fuel to the second electrode, and at least one of theelectrodes consisting of an oxygenion conducting metal oxide and, atleast partially dissolved therein, a metal oxide selected from the groupconsisting of iron oxide, manganese oxide, cobalt oxide, vanadium oxide,titanium oxide, chromium oxide, zinc oxide, titanium oxide-iron oxide,and zinc oxide-iron oxide.

In such a fuel cell, a gaseous oxidant containing molecular oxygen issupplied during cell operation to the electrode which functions as thecathode. Fuel is supplied during cell operation to the electrodefunctioning as the anode. Either or both of these electrodes is a mixedconducting oxide electrode as described above.

In the preparation of the composite article shown in FIGURES 1, 2 and 3of the drawing, the solid oxygen-ion electrolyte of stabilized zirconiais prepared from zirconia powder to which has been added approximately14 molecular percent calcium oxide. The material is formed into a hollowtubular member, a container or a at plate shown in FIGURES 1, 2 and 3.If desired, the solid stabilized Zirconia can be purchased commercially.The mixed conducting oxide electrode is formed on one surface or a pairof such electrodes are formed on both surfaces of the solid stabilizedzirconia electrolyte to provide a composite article. For example,zirconia powder, which has been stabilized by the addition of 13.75weight percent of yttria, has added thereto 2 weight percent to 20weight percent of iron oxide powder, such as Fe3O4, which powders arethen mixed and ground together. This mixture is then calcined at 1350 C.which results in a partially sintered product. This partially sinteredproduct is reground to provide a powder. The reground powder is madeinto a slurry with a 5 percent aqueous solution of polyvinyl alcohol.

The slurry is then painted onto the inner surface, outer surface, oronto both surfaces of the stabilized zirconia electrolyte, such as thehollow tubular member shown in FIGURE 1 of the drawing. An electricallycontinuous network of metallic electrical conductors might be placedadjacent to one or both surfaces of the stabilized zirconia electrolyteso that it is embedded in the slurry to promote collection of currentfrom the electrodes of a complete fuel cell. An assembly of the solidstabilized zirconia electrolyte with the slurry painted thereon is thendried, as for example, by infra-red heating to remove moisture and toform a composite article. This composite article is then assembled withother components as described above to form a fuel cell 19 as shown inFIGURE 4 of the drawing.

Heat, such as waste heat, is supplied from a source (not shown) to fuelcell 19 to raise the temperature of electrolyte 11 and electrodes 12 ofcell 19 to a preferred temperature of 1350 C. to sinter the compositearticle. If desired, such heating is done prior to assembly of fuel cell19. If the sintering of composite article 10 is done in cell 19, thetemperature is, if desired, changed for fuel cell operation to adifferent temperature above 600 C. A gaseous oxidant containingmolecular oxygen, such as air, is supplied through tube 22 to the airpassage between cathode 12 and the interior surface of tube 19. Agaseous fuel such as hydrogen is supplied through tube 23 to the chamberdefined by the interior of electrode 12, the anode. The reaction at thesurface of the cathode is as follows:

(l) Oz-i-4e 20: The oxygen ions `move through the cathode 12 andelectrolyte 11 to combine with hydrogen in accordance with the followingreaction at the surface of the anode:

through the opening at the right-hand end of the cell to the atmosphere.

Examples of electrode materials embodying our invention are as follows:

A plurality of mixed conducting oxide electrodes in disc form wereprepared which consisted of an oxygenion conducting metal oxide ofstabilized zirconia and a metal oxide selected from the above previouslydiscussed group. Each disc was made by dry mixing the respective oxidesand pressing without a binder in a one-inch diameter die. Each disc wasthen fired in air at an elevated temperature. In Table I below, thecomposition in weight percent of each of these discs is set forth. TableI recites also that each of these discs was air tired and sets forth thetemperature of the air firing for each disc. Discs Nos. l, 2 and 6 wereelectroded with sputtered platinum.

TABLE I Composition, Weight Percent Air Fired, C.

(l) 76.5 Zr02, 13.5 CaO, F6304 1,450 (2) 77.5 ZrOz, 12.5 YzOa, 10 F63041,350 (3) 77.5 ZrOz, 12.5 Y203, 10 F9304.. 1, 450 (4) 69.0 ZrOz, 11YzOg, 20 F6304... 1, 350 (5) 69.0 Zr02, 1l YzOg, 20 F0304... 1,450 (6)69.7 ZrOz, 13 6 CaO, 16 7 F9304 1,450 (7) 69.7 ZrOQ, 13.6 CaO, 16.7C00... 1,800 (8) 69.7 ZrOQ, 13.6 CaO, 16.7 ZnO 1,800 (9) 67.5 Zr02, 15Y203, 7.5 TiOz, 10 Fe304 1, 350 (10) 76.0 ZrOz, 10 YgOa, 3.5 ZnO, 10.5F0504 1,350

The above pressed discs Nos. l, 2 and 6 were then each tested todetermine that it was a mixed conducting oxide, that is an oxide whichexhibits both ionic and electronic conductivity. These discs were testedby a method devised for making ionic transport measurements which methodis set forth in an article entitled Bulk Versus Surface Conductivity ofMgO Crystals by Dr. S. P. Mitoff on pp. 2561 and 2562 of the Oct. 15,1964 issue of The Journal of Chemical Physics, vol. 41, No. 8. Each ofthese tested discs exhibited an ionic transport number less than onewhich showed that each of these discs was a mixed conducting oxide whichhad both ionic and electronic conductivity.

A composite article is prepared in accordance with FIGURE 1 of thedrawing wherein a solid oxygen-ion electrolyte member of zirconiastabilized with approximately 14 molecular percent calcia has adherentelectrodes on opposite surfaces thereof. Each of the electrodes, whichis a mixed conducting oxide and therefore exhibits both ionic andelectronic conductivity, consists of 77.5 weight percent zirconia, 12.5weight percent yttria, and 10 weight percent iron oxide, Fe304. Thesolid stabilized oxygen-ion conducting member of stabilized zirconia iseither prepared or purchased commercially. The above electrodecomposition is mixed together and has added to it a 5 percent aqueoussolution of polyvinyl alcohol to provide a slurry. The slurry is thenpainted on both sides of the electrolyte member. Infra-red heating isemployed initially to dry the assembly of the electrolyte member with acoating on each of its surfaces. This assembly is then fired in air at1350 C. for a period of time sufficient to density the electrodestructure. This sequence of steps provides a composite article having asolid oxygen-ion conducting member of stabilized zirconia, and anadherent electrode on opposite surfaces of the member, each of theelectrodes consisting of stabilized zirconia, and 10 weight percent ofiron oxide partially dissolved in the stabilized zirconia.

This composite article is then employed as theelectrode-electrolyte-electrode body for a fuel cell vassembly 6 inaccordance with FIGURE 4 of the drawing. This cell is operated byemploying a fuel such as hydrogen gas and a gaseous oxidant such asoxygen in accordance with the abovedescribed operation of this cell.

While other modifications of this invention and variations thereof whichmay be employed within the scope of the invention have not beendescribed, the invention is intended to include such as may be embracedwithin the following claims.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In an electrical device for operation at temperatures in excess ofabout 600 C., said device comprising a solid anode layer and a solidcathode layer as the electrodes separated by and in direct contact witha layer of a sintered solid oxide-ion electrolyte selected from thegroup consisting of stabilized zirconia and doped thoria, theimprovement wherein at least one electrode is (a) substantiallynon-porous, (b) tightly adherent to the electrolyte layer and (c)consists of a solid oxide-ion electrolyte selected from theabove-mentioned group having at least partially dissolved therein ametal oxide selected from the group consisting of iron oxide, manganeseoxide, cobalt oxide, vanadium oxide, titanium oxide, chromium oxide,zinc oxide, titanium oxideiron oxide and zinc oxide-iron oxide, saidmixture exhibiting both ionic and electronic conductivity.

2. The improvement substantially as recited in claim 1 wherein theoxide-ion electrolyte is stabilized zirconia and the metal oxide mixedtherewith is iron oxide in an amount ranging from between 2 and 20weight percent of the mixture.

3. In a fuel cell for operation at temperatures in excess of about 600C., said fuel cell comprising a solid r anode layer and a solid cathodelayer as the electrodes separated by and in direct contact with a layerof a sintered solid oxide-ion electrolyte selected from the groupconsisting of stabilized zirconia and doped thoria, the improvementwherein at least one electrode is (a) substantially non-porous, (b)tightly adherent to the electrolyte layer and (c) consists of a solidoxide-ion electrolyte selected from the above-mentioned group having atleast partially dissolved therein a metal oxide selected from the groupconsisting of iron oxide, manganese oxide, cobalt f oxide, vanadiumoxide, titanium oxide, chromium oxide,

zinc oxide, titanium oxide-iron oxide and zinc oxide-iron oxide, saidmixture exhibiting both ionic and electronic conductivity.

4. The improvement substantially as recited in claim 1 wherein theoxide-ion electrolyte is stabilized Zirconia and the metal oxide mixedtherewith is iron oxide in an amount ranging from between 2 and 20weight percent of the mixture.

References Cited UNITED STATES PATENTS 2,535,526 12/1950 Ballard et al.106-57 3,160,527 12/1964 Hess 136-86 3,281,273 10/1966 Oser 136-863,300,344 l/1967 Bray et al. 136-86 FOREIGN PATENTS 626,316 S/l96lCanada.

Kiukkola et al.: Measurement on Galvanic Cells Involving SolidElectrolytes, August 1956, Dept. of Technology, MIT, Cambridge, Mass.

ALLEN B. CURTIS, Primary Examiner.

